Milled MgH2, NaAlH4, MgH2–10NaAlH4, MgH2–30NaAlH4, MgH2–50NaAlH4, and MgH2–2Ni-10NaAlH4 samples were prepared by milling in hydrogen atmosphere (reactive mechanical milling, RMM). Decomposition temperatures of milled MgH2, NaAlH4, MgH2–10NaAlH4, and MgH2–30NaAlH4 were examined in a Sieverts-type hydrogen-absorption and release apparatus, in which the hydrogen pressures were kept constant during hydrogen absorption or release. As the content of NaAlH4 in the sample increased, the temperature at the highest peak in the ratio of increase in Hr to increase in T, dHr/dT, versus T curve decreased. The higher content of NaAlH4 is believed to have made effects of reactive mechanical milling stronger. Effects of the NaAlH4 content on the temperatures of intermediate reactions were studied for MgH2 – NaAlH4 composites. Phase formation in the cycled MgH2–50NaAlH4 were investigated. The hydrogen-storage properties of MgH2–30NaAlH4 were compared with those of Mg-based alloys in which oxide, halides, or fluoride was added.